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**Tangents to a parabola**

This is a parabola as the track of all objects with equal
distance to F and the line AB. The two tangents to the parabola belonging to A
and B intersect at a point, which projects to the midpoint of the line segment
AB. The proof is hidden. Right click to the construction and enable the display
of all hidden objects.

### Technical Information

To hide like this is a simple trick to let the user think
before he sees the full construction. Another idea is to use a separate web
page. Of course, unnecessary details (like the construction of the track) should
be completely hidden.